Select the joined rectangles and the O-shape and Combine Shapes > Subtract Shapes (Arrange menu). This operation will remove the portion of the
rectangles that were overlapping the O-shape. (It also removes the rectangles).

NOTE: Hold down the Shift key to select multiple objects.

Using the Ellipse Tool
, create a 20 pixel diameter circle. Center align it vertically with the O-shape and between the sides of the O-shape as shown.

Drag and duplicate three more circles in a counterclockwise direction as shown.